This invention relates to a method for preparing a dye sensitized dichromated gelatin hologram and to the hologram prepared therefrom.
One of the more important discoveries emerging from the development of the laser is a recording technique known as holography. In essence, this technique records, and later reconstructs the amplitude and phase distributions of a coherent wave disturbance.
The technique is widely used as a method of optical image formation, and in addition has been successfully used with acoustical and radio waves.
The fundamentals of holography are known; and the technique, generally, is accomplished by recording the pattern of interference between the unknown "object" wave of interest and a known "reference" wave. In general, the object wave is generated by illuminating the (possibly three-dimensional) subject of concern with a highly coherent beam of light, such as supplied by a laser source. The waves reflected from the object strike a light-sensitive recording medium, such as photographic film or plate. Simultaneously a portion of the light is allowed to bypass the object, and is sent directly to the recording plane, typically by means of a mirror placed next to the object. Thus incident on the recording medium is the sum of the light from the object and a mutually coherent "reference" wave.
While all light-sensitive recording media respond only to light intensity (that is, power), nonetheless in the pattern of interference between reference and object waves there is preserved a complete record of both the amplitude and the phase distributions of the object wave. Amplitude information is preserved as a modulation of the depth of the interference fringes, while phase information is preserved as variations of the position of the fringes.
The photographic recording obtained is known as a hologram (meaning a "total recording"); this record generally bears no resemblance to the original object, but rather is a collection of many fine fringes which appear in rather irregular patterns. Nonetheless, when this photographic transparency is illuminated by coherent light, one of the transmitted wave components is an exact duplication of the original object wave. This wave component therefore appears to originate from the object (although the object has long since been removed) and accordingly generates a virtual image of it, which appears to an observer to exist in three-dimensional space behind the transparency. The image is truly three-dimensional in the sense that the observer must refocus his eyes to examine foreground and background, and indeed can "look behind" objects in the foreground simply by moving his head laterally.
Also generated are several other wave components, some of which are extraneous, but one of which focuses of its own accord to form a real image in space between the observer and the transparency. This image is generally of less utility than the virtual image because its parallax relations are opposite to those of the original object.
Because of the emerging importance of holograms, a considerable research effort has been directed toward the mechanics of recording the image and the media utilized to effect and display the image. A typical media for recording the hologram is composed of an aqueous gelatin system such as pyridine-di-chromate or ammonium dichromate. The efficacy of the dichromated gelatin system is based upon the difference in swelling between the exposed and unexposed gelatin. The difference in swelling is induced photochemically by cross-linking of the gelatin through photolytic decomposition products of the dichromate sensitizer which renders the gelatin insoluble to a degree dependent on total exposure. The resultant image is developed by removal of gelatin not previously cross-linked as well as by the shrinkage and splitting or cracking of the gelatin.
Although dichromated gelatin systems have been proven to be satisfactory for use as a holographic material, such systems suffer certain drawbacks because their spectral sensitivity is limited to wavelengths of less than 520 nanometers. In an attempt to overcome this problem, however, it has been discovered that the addition of a dye sensitization material extends spectral sensitivity of the dichromated gelatin system to an unexpected degree.